The invention relates to a receiver for selectively receiving an FM-AM type FM stereo multiplex broadcasting and an FM-FM type television sound multiplex broadcasting.
With recent practical realization of a television (TV) sound multiplex broadcasting in Japan, a receiver which permits reception of both TV sound multiplex broadcasting and the conventional FM stereo multiplex broadcasting has been desired. Both types of broadcasting employ the FM system for main carrier modulation but different modulation systems for subcarrier modulation. In the TV sound multiplex broadcasting, the subcarrier is frequency-modulated while, in the FM stereo multiplex broadcasting, it is amplitude-modulated as is well known. This makes it difficult to use a common demodulator circuit for the TV sound multiplex signal and FM stereo multiplex signal. For this reason, it is a common practice that separate demodulator circuits are provided for demodulating both of the multiplex signals. In this respect, a conventional receiver needs a number of circuit components, thus increasing the cost and size of the receiver.
In this type receiver, in order to indicate whether the TV sound broadcasting is the multiplex broadcasting or not there is a need for provision of a circuit for detecting the multiplex broadcasting. In the conventional receiver, the TV sound multiplex-broadcasting detecting circuit is provided separately from the FM stereo multiplex demodulator circuit. Also in this respect, the cost and size of the receiver inevitably increase. Television sound multiplex broadcasting is categorized into a stereo broadcasting and a dual sound broadcasting, e.g. a bilingual broadcasting. The receiver needs therefore a circuit for automatically switching the receiving circuit depending on the stereo broadcasting or the dual sound broadcasting and a visual indication circuit therefor. In the conventional receiver, such circuits are provided independently of the demodulator circuit for FM stereo multiplex broadcasting. This also leads to the same problems as mentioned above.